if authorities hadn't stopped tsarnaev and his older brother, tamerlan. >> "we have reason to believe, based upon the evidence that was found at that scene, the explosions, the explosive ordinance that was unexploded and the fire power that, they had that they were going to attack other individuals." >> reporter: in 2011, the f- b-i says it interviewed tamerlan, after russian authorities alerted the u-s government. the elder tsarnaev then made a six-month trip to russia in 2012, but his exact whereabouts and activities are unclear. >> "the ball was dropped in one of two ways, the fbi missed a lot of things is one potential answer, or our laws do not allow the fbi to follow-up in a sound, solid way." tamerlan's youtube channel linked to a video featuring a well-known jihadist. who was killed by russian forces last december. there is no evidence tamerlan was associated with any international jihadist groups. >> "you feel a little safer right now, knowing that the ones that did it aren't here anymore, and now it's just about trying to move on and put the pieces together and get ready for next year."